To install a theme, download one from below and copy the files to the "apps" folder, just like an homebrew application.
As soon you load it from the Homebrew Channel, the theme will be installed.

Note: You must have the Homebrew Channel 1.0.7 or higher in order to use themes. Update your Homebrew Channel with the HackMii Installer.

It is stored in the channel's savegame on the Wii, so you can safely remove your SD card or USB mass storage device without losing the theme.
If you delete the savegame from the Wii, your theme will be lost. You can also send the theme.zip using Wiiload.

To create your own theme, extract a theme.zip and replace the graphics. They need to have the same dimensions, filenames and filetypes as the orginals, else the images will be ignored and replaced by the original (Dark Waters) graphics. The theme.xml contains information about colors for the text (globally used) and the progress bar, see also Theme.xml Creator. When you're done, you can put the images and theme.xml back into the theme.zip, maybe add a meta.xml and icon.png and upload in here.

For those having trouble you can use this file as a template. Also Newlife has created an application called ThemeHBC as an alternative way to create themes.

Don't worry about taking a screenshot, a qualified volunteer will handle it. As for HBC 1.1.0, you can take screenshots by connecting a Nunchuk and pressing Z+C (in that order); the screenshot will be written to the SD card as hbc-001.png and up. This is the preferred method. Other theme screenshots taken with ThemeShooter are referred to as "Themeshot's".

Rules

Do not upload themes containing copyrighted material

Use only images you created or that are explicitly(!) released to be re-used in other works. If you are unclear about whether you are allowed to use an image, don't use it. Just because you found an image on the internet, it doesn't mean you are allowed to use it.

Do not bend this rule: Cropping, cutting up, changing the colors or filtering an image does not void its copyright. "Fair use" is not applicable either (and a US-only exemption to copyright anyway). For the time being, Logos are considered somewhat safe, and can be used (if you really, really have to - it's not the pinnacle of creativity to slap a huge company or game logo in the background).

Do not upload themes containing only a background image

Please do at least some customization of the GUI images. Although background-only themes are possible, they are rather pointless.

Keep your descriptions short

When uploading and adding your theme to this page please keep your descriptions short, we don't want to hear the whole story just the basics. If you want to tell your whole story you can put it in the theme's meta.xml file. To avoid clutter and keep this page organized we ask that you please follow this new limit of four lines, this means that you must use the "Show preview" button to make sure that it doesn't go past your theme's screenshot.